1. Field of the Invention
The present invention relates generally to a device for converting an analog signal into a digital signal in an image scanning device. More particularly, the present invention relates to an image scanning device for converting an image signal scanned in analog form into an image signal in digital form corresponding to an operation mode, and an image scanning method thereof.
2. Description of the Related Art
The image scanning device is included in a digital image processing apparatus such as a scanner, a facsimile, and a digital copier, which scans the image of a given document and processes the image in the form of a digital signal. FIG. 1 is a block diagram illustrating a conventional image scanning device.
As shown in FIG. 1, the image scanning device scans the image of a document disposed on a document plate 11 by an image sensor 13. The image signal scanned in the form of an analog signal is converted into a digital image signal by an analog-to-digital (A/D) conversion unit 15. An image processing unit 17 performs the signal processing such as enlargement and reduction operations on the digital image signal and has the processed signal stored in the storage unit 19.
FIG. 2 is a block diagram illustrating the A/D conversion unit 15 of a conventional image scanning device.
The A/D conversion unit 15 sets an upper reference voltage ET and a lower reference voltage EB based on a color mode having a large accumulated light amount, and converts the analog image signal output from the image sensor 13 into the digital image signal based on the reference voltages ET and EB. If the digital image signal has an 8 bit level, the lower reference voltage EB is a 0 level and the upper reference voltage ET is a 255 level. The 8 bit level of digital signal is output, which corresponds to the voltage value of the image signal input based on the voltage values corresponding to 0 through 255 levels.
When using the fixed upper reference voltage ET and the lower reference voltage EB, the correct picture quality image is not obtained in the color mode and the monochrome mode. Specifically, a minimum scanning time of the monochrome mode scanned by a single light source is faster than that of the color mode scanned by Red (R), Green (G), and Blue (B) light sources, and hence the accumulated light amount in the color mode is inversely larger than that in the monochrome mode.
When applying the same upper reference voltage ET and lower reference voltage EB to the color mode and the monochrome mode each having different accumulated light amounts for converting the analog signal into a digital signal, the printed document does not have the highest image quality.